1. Field of the Invention
This invention relates to the production of metal alkyl carbonates soluble in C.sub.1 -C.sub.3 monhydric alcohols, useful as carboxylating agents.
2. Description of the Prior Art
Metal alkyl carbonates are known in the prior art as useful carboxylating agents for nitroalkanes and ketones, as shown in J. Am. Chem. Soc., Volume 81, pages 505 to 506, 2598 to 2599 (1969), Ann. New York Acad. Sci., Volume 88, pages 332 to 340 (1970), and "Reagents for Organic Synthesis", pages 631 to 633 by Fieser and Fieser (1967). The most important metal alkyl carbonate for use in carboxylation reactions is magnesium methyl carbonate (MMC) which is not normally isolated, but is generally prepared in methanol solution, the methanol then being replaced by a polar aprotic organic solvent in which carboxylation reactions can occur.
The methods of preparation of metal alkyl carbonates, soluble in monohydric alcohols containing 1 to 3 carbon atoms, such as MMC, described in the above references, either involve starting with expensive and hygroscopic metallic alkoxides such as magnesium methoxide and dissolving them in the appropriate alcohol, or forming them in situ by reacting an active metal, e.g. magnesium, with the appropriate alcohol, in which hydrogen is evolved as a by-product. Carbon dioxide is bubbled through the alcoholic solution of metallic alkoxide, prepared by either method, to form the metal alkyl carbonate. The alcohol is then replaced with a polar aprotic solvent, such as dimethylformamide. However, these methods suffer the disadvantages of either using expensive and moisture-sensitive starting reagents such as metallic alkoxides or the hazardous formation of the starting reagent in which flammable by-product hydrogen is evolved.
Another known procedure for producing metal alkyl carbonates is exemplified in U.S. Pat. No. 3,445,497 (1969, Anderson et al. to Signal Oil) in which a tertiary organic amine is reacted with carbon dioxide and alcohol to form an adduct, trialkylammonium alkyl carbonate, which is then reacted with an alcohol-soluble inorganic metal salt to form the metal alkyl carbonate in a reversible reaction. This process is useful for forming alcohol-insoluble metal alkyl carbonates in which the reversible reaction is carried to completion due to precipitation of the product from the reaction medium but is unsuitable for forming high yields of alcohol-soluble metal alkyl carbonates such as magnesium methyl carbonate (MMC) for use in subsequent carboxylation reactions. In the case of alcohol-soluble metal alkyl carbonates such as MMC, the above process is unsuitable since the reagent will not ordinarily function as a carboxylating agent in the presence of methanol, and cannot be transferred to methanol-free dimethylformamide solution without reverting to the original metal salt. This is due to the failure to remove the by-product acid present in solution as its trialkylammonium salt.
We have found that by-product acid, present as trialkylammonium salt, adversely interferes with the high yield production of alcohol-soluble metal alkyl carbonates, particularly MMC, by reversing the equilibrium reaction, and also that by-product acid causes an acid-catalyzed cleavage of the alcohol soluble metal alkyl carbonate when replacing the alcohol with a polar aprotic solvent prior to the carboxylation reaction.
It is, therefore, an object of this invention to provide a process for preparing metal alkyl carbonates, soluble in monohydric alcohols containing 1 to 3 carbon atoms, in high yield from inexpensive reagents, not involving hazardous process steps, which are free of by-product acid.